Any builder who says that he hasn?t had one is suffering for selective memory ? or isn?t really a builder! These are the times that make you wonder if you?re ever going to finish, and sometimes even if yu want to continue. I?m talking about those kind of days where you spend lots of hours, cut and shape a lot of aluminum ? and have nothing to show for it in the end. Yes, it happens to everyone, and it?s time we admit it so that the guys experiencing one for the first time don?t get too discouraged!
Here?s an example. Building the RV-3 is an adventure in detailed design ? there are lots of details on any airplane of course, but in the -3, most of them are left up to the builder?s imagination. Such, amazingly, is the trim tab hardware. Yes, building it to the plans is probably simpler than putting in electric trim, but even the ?stock? system requires a lot of imagination. So I have been sort of working on the trim on and off for a year or so. I knew we wanted to go electric, and we had Randy Lervold?s airplane as an example ? he mounted a servo on the aft fuselage deck and ran a short cable to the elevator. I had found some interesting-looking miniature servos from Firgelli that I considered, and even obtained, but after trying to fit it in, I became dubious of some plastic parts in something as critical as pitch trim (I am still using the sub-miniature Firgellies for aileron and rudder tabs). By this time, I had already cut some holes in the elevator and tried to build some brackets, so I was beginning to make mods to mods.
So back I went to Randy?s idea. The geometry of the cable and control horn on the tab weren?t working out very well ? I had already made four tabs to get enough clearance and throw to keep the cable end from binding. Then the other night I had the thought that if I put a little bend in the end of the cable (I did this to the pushrod on my -8), I could have clearance AND control), so I put the cable end in a vice and gave it a little whack with the hammer, and it snapped right off ? completely brittle! OK?well, where was that Firgelli again? Hmmm?.maybe if I tried just a little harder?..
With several months behind me to give a better perspective I saw a possible way to make it work, and began building yet another bracket idea ? spent a couple of hours on it, fabricated an end for the servo out of steel, and was really making progress ? right up until the time that I didn?t have it lined up quite right doing a motion test, jammed it slightly ? and that plastic end I was worried about snapped off. Hmmm?well, that left me a clean end which I could drill for a stainless screw, and then I wouldn?t have that trouble again! But after spending a half hour prepping that little fix, I reinstalled it in my mock-up and tried it again ? only to find that the unit reached the end of travel and free-wheeled?and wouldn?t drive back the other way. Close examination showed that a couple of little plastic tabs that held the shaft to the motor had also stripped out. OK, so my first instincts were correct ? not up to par for pitch trim in an airplane!
So?back to Randy?s plan. I looked at the end that had snapped off, and realized that it was much longer than I needed ? and I had the ability to tap it, so I put some threads back onto work with until a new unit came from ACS. And?.that was about all I got done, because it was getting late, and I had an early show-up time in a sim the next day. Sum total of accomplishments (toward getting the airplane finished)? Zero, nada, nothing. But then again, like Edison and the light bulb project, I had learned a number of things that WOULDN?T work, so maybe it was progress after all?..
(Addendum ? the next day in the shop proved incredibly fruitful, with completion of the entire trim system using Randy?s concept and photos as a guide. Now hopefully, the servo won?t be too close to the magnetometer?.and so it goes?.)
Paul
Here?s an example. Building the RV-3 is an adventure in detailed design ? there are lots of details on any airplane of course, but in the -3, most of them are left up to the builder?s imagination. Such, amazingly, is the trim tab hardware. Yes, building it to the plans is probably simpler than putting in electric trim, but even the ?stock? system requires a lot of imagination. So I have been sort of working on the trim on and off for a year or so. I knew we wanted to go electric, and we had Randy Lervold?s airplane as an example ? he mounted a servo on the aft fuselage deck and ran a short cable to the elevator. I had found some interesting-looking miniature servos from Firgelli that I considered, and even obtained, but after trying to fit it in, I became dubious of some plastic parts in something as critical as pitch trim (I am still using the sub-miniature Firgellies for aileron and rudder tabs). By this time, I had already cut some holes in the elevator and tried to build some brackets, so I was beginning to make mods to mods.
So back I went to Randy?s idea. The geometry of the cable and control horn on the tab weren?t working out very well ? I had already made four tabs to get enough clearance and throw to keep the cable end from binding. Then the other night I had the thought that if I put a little bend in the end of the cable (I did this to the pushrod on my -8), I could have clearance AND control), so I put the cable end in a vice and gave it a little whack with the hammer, and it snapped right off ? completely brittle! OK?well, where was that Firgelli again? Hmmm?.maybe if I tried just a little harder?..
With several months behind me to give a better perspective I saw a possible way to make it work, and began building yet another bracket idea ? spent a couple of hours on it, fabricated an end for the servo out of steel, and was really making progress ? right up until the time that I didn?t have it lined up quite right doing a motion test, jammed it slightly ? and that plastic end I was worried about snapped off. Hmmm?well, that left me a clean end which I could drill for a stainless screw, and then I wouldn?t have that trouble again! But after spending a half hour prepping that little fix, I reinstalled it in my mock-up and tried it again ? only to find that the unit reached the end of travel and free-wheeled?and wouldn?t drive back the other way. Close examination showed that a couple of little plastic tabs that held the shaft to the motor had also stripped out. OK, so my first instincts were correct ? not up to par for pitch trim in an airplane!
So?back to Randy?s plan. I looked at the end that had snapped off, and realized that it was much longer than I needed ? and I had the ability to tap it, so I put some threads back onto work with until a new unit came from ACS. And?.that was about all I got done, because it was getting late, and I had an early show-up time in a sim the next day. Sum total of accomplishments (toward getting the airplane finished)? Zero, nada, nothing. But then again, like Edison and the light bulb project, I had learned a number of things that WOULDN?T work, so maybe it was progress after all?..
(Addendum ? the next day in the shop proved incredibly fruitful, with completion of the entire trim system using Randy?s concept and photos as a guide. Now hopefully, the servo won?t be too close to the magnetometer?.and so it goes?.)
Paul
